It depends, is the horse barefoot and due to be trimmed, it could just be growth that will be trimmed off. There are tons of factures that will cause cracks. I would first discuss it with your farrier first since he deals with your horses feet and knows them the best to see if he is concerned about it at all. Then go from there. My horses wear front shoes only and are barefoot in the back. Their back feet have small cracks but nothing to worry about. So I would talk with your farrier first, and then a vet if needed. But I would not be filling a crack. You could make it worse.